In recent years, there have been proposed a number of amorphous semiconductor films comprised of amorphous silicon materials compensated with hydrogen atoms or/and halogen atoms such as fluorine or chlorine atoms (hereinafter referred to as "A-Si(H,X)") as element members to be used in semiconductor devices, electrophotographic photosensitive devices, image input line sensors, image pickup devices, photovoltaic devices, and other than these, various electronic devices, optical elements, and the like. Some of such films have been put to practical use.
It is known that such semiconductor deposited films may be obtained by means of a plasma CVD process, that is, a process of forming a deposited film on a substrate of glass, quartz, heat-resistant resin, stainless steel or aluminum by decomposing a film-forming raw material gas with the action of glow discharge energy of direct current, high frequency energy, microwave, etc. A variety of apparatus for practicing such process have been proposed. In recent years, the public attention has been focused on the plasma CVD process by the microwave glow discharge decomposition technique, and various studies have been made in order to practice said process on the industrial scale. The MW-PCVD method is remarkably advantageous over other methods in that film deposition rate is high and utilization efficiency of a film-forming raw material gas is high.
A typical CVD apparatus by the microwave glow discharge decomposition process is disclosed in Japanese Laid-open patent application 59-078528. While the Japanese Laid-open patent application 61-283116 discloses an improved microwave plasma CVD apparatus, it also discloses a process of performing film deposition while controlling the potential of plasma in the discharge space by applying a desired voltage through an electrode disposed in the discharge space to control ion bombardment of depositing species. This literature describes that the property of a film to be formed according this process is substantially improved.
The deposited film-forming apparatus by such known microwave plasma CVD process is typically of the constitution shown in FIG. 1.
The apparatus shown in FIG. 1 is of the type that a single substrate is placed.
In FIG. 1, reference numeral 401 stands for a reaction vessel having a vacuum tight structure. Reference numeral 402 stands for a dielectric window made of a material capable of efficiently transmitting a microwave power and of maintaining vacuum-tightness such as quartz glass, alumina ceramics or the like. Reference numeral 403 stands for a microwave transmitting portion comprising a metallic waveguide connected to a microwave power source (not shown) through a stub tuner (not shown) and an isolator (not shown). Reference numeral 404 stands for an exhaust pipe which is open into the vacuum vessel 401 through one end thereof, whereas the other end thereof is connected to an exhaust device (not shown). Reference numeral 405 stands for a substrate on which a deposited film is to be formed, and reference numeral 406 stands for a discharge space. Reference numerals 409 and 410 respectively stand for a power source and an electrode respectively for controlling the potential of plasma.
The formation of a deposited film in such conventional deposited film-forming apparatus is carried out in the following manner in the case of using the apparatus shown in FIG. 1.
That is, the inside of reaction vessel 401 is evacuated by means of a vacuum pump (not shown) to bring the inner pressure of the reaction vessel 401 to 1.times.10.sup.-7 Torr or below. Subsequently, the substrate 405 is heated to and maintained at a temperature suitable for the formation of a desired deposited film by the use of a heater installed within a substrate holder 407. Film-forming raw material gas, for example silane gas (SiH.sub.4) in the case of forming an amorphous silicon deposited film, is introduced into the reaction vessel 401. Simultaneously with this, the microwave power source (not shown) is switched on to generate a microwave having a frequency of not less than 500 MHZ, preferably 2.45 GHz, followed by transmission through the waveguide 403 and the dielectric window 402 into the reaction vessel 401. In this way, the gas in the reaction vessel 401 is excited and dissociated by means of microwave energy to cause the formation of a deposited film on the surface of the substrate 405.
According to the conventional deposited film-forming apparatus, it is possible to form a relatively thick photoconductive film at a relatively high deposition rate.
However, as for such conventional apparatus, in the case where the formation of a large area film with uniformity in characteristics all over the film is required, for example, as in the case of an electrophotographic photosensitive member, it is not sufficient to meet the reqirement; and other than this, there are other disadvantages to be eliminated particularly in economy of operation.
As for the process of forming a deposited film, U.S. Pat. No. 4,504,518 describes a microwave plasma CVD process with advantages that a relatively high raw material gas utilization efficiency and a relatively high deposition rate can be attained. The technique described in this literature aims at obtaining a good quality deposited film at a low pressure of 0.1 Torr or below and at a high deposition rate by the microwave plasma CVD process.
Likewise, Japanese Laid-open application 60-186849 proposes a technique in order to improve the raw material gas utilization efficiency in the microwave plasma CVD process, and this technique is to establish an inner chamber (that is, discharge space) so as to circumscribe the microwave energy introducing portion.
As for the microwave plasma CVD apparatus, Japanese Laid-open applications 63-57779 and 63-230880 propose respectively an improvement in the raw material gas supply portion. The improvement concerns the introduction of a raw material gas into the discharge space through the space between the adjacent cylindrical substrates using a comb shaped or triangle pole-like shaped gas supply means, and it makes it possible to efficiently introduce the raw material gas into the plasma generation region and to improve the deposition rate of a deposited film.
In accordance with these conventional microwave plasma CVD processes, it is possible to form a relatively thick photoconductive material at a relatively high deposition rate and with a relatively high raw material gas utilization efficiency.
An example of the foregoing deposited film-forming apparatus is of such constitution schematically shown in FIGS. 2 and 3. FIG. 2 is a schematic cross section view illustrating the constitution of the apparatus. FIG. 3 is a schematic cross section view taken along line B-B' in the apparatus shown in FIG. 2.
Referring to FIGS. 2 and 3, reaction chamber 501 is provided integrally with exhaust pipe 504 at the side face thereof, and the other end of the exhaust pipe 504 is connected to an exhaust device. Waveguide 503 is mounted at each of the upper and lower faces of the reaction chamber 501, and one end of each of the two waveguides 503 is connected to a microwave power source (not shown). The other end of each of the two waveguides 503 on the side of the reaction chamber 501 is hermetically provided with a dielectric window 502. Six cylindrical substrates 505 are arranged in parallel with each other so as to circumscribe the central portion of the reaction chamber 501. Each of the cylindrical substrates 505 is held by a rotary shaft 508, and is designed such that it is heated by a heater 507. The cylindrical substrate 505 is rotated around the central axis in the generatrix direction by actuating a motor 509 to rotate the rotary shaft 508 through a reduction gear 508. The space circumscribed by the cylindrical substrates 505 and the opposite dielectric windows 502 in the reaction chamber 501 is discharge space 505. A bias electrode 552 is installed substantially near the central portion of the discharge space 506 such that it is substantially in parallel to each of the cylindrical substrates 505. The bias electrode 552 is connected to a bias power source 512 through a cable 513. A raw material gas feed pipe 551 is arranged in the space between each pair of the adjacent cylindrical substrates 505. Each of the raw material gas feed pipes 551 is of a comb shape and serves to supply a raw material gas into the discharge space 506.
Upon forming a deposited film, for example for an electrophotographic photosensitive member, the reaction chamber 501 is firstly evacuated to a vacuum of less than 1.times.10.sup.-7 Torr, then the cylindrical substrates are heated to and maintained at a desired temperature by means of the heaters 507. Thereafter, raw material gases, for example, silane gas, etc. in the case of forming an amorphous silicon deposited film, are supplied into the reaction chamber 501 through the raw material gas feed pipes 551. Simultaneously with this, a microwave having a frequency of not less than 500 MHz, preferably 2.45 GHz is introduced into the reaction chamber 501 through the waveguide 503 and the dielectric window 502. As a result, glow discharge is caused in the discharge space 506 to excite and dissociate the raw material gases, whereby forming a deposited film on each of the cylindrical substrates 505. In this case, the motors 509 are actuated to rotate the cylindrical substrates and by this, the deposited film is formed on the entire surface of each of the cylindrical substrates 505.
However, there are some disadvantages necessary to be eliminated in the conventional microwave plasma CVD technique as will be described hereafter.
That is, in the case of externally supplying a raw material gas, as the raw material gas is introduced into the plasma region, decomposition and ionization are caused from one to another, wherein the radical density and the ion density of the raw material gas in the plasma in the upstream side (the side where the raw material gas is introduced) are markedly different from those in the downstream side (the exhaust means side).
Because of this, the volume and the intensity of ion bombardment on the substrate in the upstream side becomes different from those in the downstream side to unavoidably cause nonuniformity in the thickness and the electric property of a deposited film formed.
In addition, as for the raw material gas, since it is externally supplied into the plasma region, part of the raw material gas is occasionally exhausted without being decomposed because it does not pass through the intense central portion of the plasma region. Thus, there is also a disadvantage to be eliminated with respect to the raw material gas.
Further, since the raw material gas is externally supplied outside the plasma region as above described, ionization thereof is performed mostly at the position remote from the bias electrode. Because of this, the ion energy is not sufficiently high as against the substrate, wherein ion bombardment against the substrate does not sufficiently occur. In this case, if the voltage of the bias electrode is heightened in order to sufficiently cause ion bombardment, abnormal discharges such as sparking and the like are caused.
On the other hand, in the case where a raw material gas feed means is installed within the portion where plasma is generated, the raw material gas always passes through the central portion of plasma and is always decomposed. The electric field in the plasma becomes disturbed by the raw material gas feed means and also by the flow of the raw material gas. Because of this, the deposited film formed on the substrate often becomes poor in uniformity of thickness and also in electrical property.
This problem becomes significant in the case where the above idea is employed in such apparatus as shown in FIGS. 2 and 3 in which a plurality of substrates are arranged so as to circumscribe the plasma region.
Further, in the case of the foregoing conventional apparatus, a problem often occurs in that since the gas feed pipes are situated to be close substrates, minute foreign matters deposited on those gas feed pipes fly up to enter into the reaction chamber and deposit on the substrates when the raw material gas is supplied into the reaction chamber. For instance, when producing an electrophotographic photosensitive member, this leads to defects on an image obtained. In addition, when forming a deposited film continuously over a long period of time, there occur such problems that the deposition rate is reduced as the film formation proceeds together with production of the minute foreign matters. These problems must be eliminated.